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Virtual machine

That the Java Virtual Machine is a virtual machine is a rather obvious observation, but it should be kept in mind. One of the consequences is that you are, in theory, writing applications for a type of machine that differs from the machine you are developing or running your applications on.

It generally does not matter whether the code runs on a 32-bit or 64-bit version of the Java Runtime Environment (JRE). The latter will probably make more memory available to the application than the 32-bit version, but the running program will not care about this difference as long as it doesn't make native operating system calls or require gigabytes of memory.

Unlike a language, such as C, where datatype sizes are dependent on the native system, Java does not have this issue (or feature, depending on your point of view). An int integer on JVM is always signed and is of 32-bit size, no matter on which computer platform or system architecture it is running.

Finally, it should be noted that each application that runs on JVM loads its own instance of JVM on system memory. This means when you run multiple Java applications at the same time, they will all have their own copy of JVM at their disposal; this also means different applications can use different versions of JVM if required for whatever reason. For security reasons, it is not suggested that you have different versions of the JDK or JRE on one system; it's usually better to have only the latest supported versions installed.